Effect of Bi on graphite morphology and mechanical properties of heavy section ductile cast iron

To improve the mechanical properties of heavy section ductile cast iron, bismuth （Bi） was introduced into the iron. Five castings with different Bi content from 0 to 0.014 wt.% were prepared; and four positions in the casting from the edge to the center, with different solidification cooling rates, were chosen for microstructure observation and mechanical properties test. The effect of the Bi content on the graphite morphology and mechanical properties of heavy section ductile cast iron were investigated. Results show that the tensile strength, elongation and impact toughness at different positions in the five castings decrease with a decrease in cooling rate. With an increase in Bi content, the graphite morphology and the mechanical properties at the same position are improved, and the improvement of mechanical properties is obvious when the Bi content is no higher than 0.011wt.%. But when the Bi content is further increased to 0.014wt.%, the improvement of mechanical properties is not obvious due to the increase of chunky graphite number and the aggregation of chunky graphite. With an increase in Bi content, the tensile fracture mechanism is changed from brittle to mixture ductile-brittle fracture.     

Effect of Cooling Rate on Microstructure and Mechanical Properties of Thin-Walled Ductile Iron Castings

This article addresses the effect of cooling rate on microstructure and mechanical properties as determined by changing molding media and section size. The research was conducted for thin-walled iron castings with 2-5-mm wall thickness and for the reference casting with 13-mm wall thickness, using different molding materials (silica sand and insulating sand “LDASC”) to achieve various cooling rates. Thermal analysis was performed to determine the real cooling rate at the beginning of the graphite eutectic solidification. In general, it was found that the predictions based on theoretical analysis of the solidification process of ductile iron are in good agreement with the experimental outcomes. Finally, the present study provides insights into the effect of cooling rate on the graphite nodule count, the ferrite fraction and mechanical properties of thin-walled ductile iron castings. The study shows that the cooling rate of thin-walled castings varies in a wide range (80-15 °C/s) when changing the wall thickness from 2 to 5 mm, accompanied by significantly changing the mechanical properties of ductile iron. The cooling rate can be effectively reduced by applying an insulating sand to obtain the desired properties of thin-walled castings practically in the whole range of ductile iron grades in accordance with the ASTM Standard.     

金属型对灰铸铁和球铁力学性能的影响研究

采用铜合金金属型和铸铁金属型浇注灰铸铁和球铁Y形试块，加入不同量的Mn、Ti、Cu和Sn，研究金属型材料和添加合金对灰铸铁和球铁抗拉强度和疲劳强度的影响。结果发现：与采用铸铁金属型浇注相比，未加合金、采用铜合金金属型浇注的灰铸铁和球铁的力学性能略高。采用铜合金铸型浇注时，由于冷速较快，使石墨和基体组织细化的作用较强，添加合金元素改善力学性能的作用较为明显，而且反复浇注时，铜合金金属型的热应力较小。因此，灰铸铁和球铁采用铜合金金属型铸造比采用铸铁金属型有利。     

Effects of forced cooling on mechanical properties and fracture behavior of heavy section ductile iron

To develop materials suitable for spent-nuclear-fuel containers, the effect of forced cooling on mechanical properties and fracture toughness of heavy section ductile iron was investigated. Two cubic castings with different cooling processes were prepared: casting A was prepared in a totally sand mold, and casting B was prepared in a sand mold with two chilling blocks placed on the left and right sides of the mold. Three positions in each casting with different solidification cooling rates were chosen. In-situ SEM tensile experiment was used to observe the dynamic tensile process. Fracture analysis was conducted to study the influence of vermicular and slightly irregular spheroidal graphite on the fracture behavior of heavy section ductile iron. Results show that the tensile strength, elongation, impact toughness and fracture toughness at different positions of the two castings all decrease with decreasing cooling rate. With the increase of solidification time, the fracture mechanism of conventional casting A changes from ductile fracture to brittle fracture, and that of casting B with forced cooling changes from ductile fracture to a mixture of ductile-brittle fracture.     

孕育处理对大断面球铁件力学性能和石墨形态的影响

基于金相组织、力学性能、拉伸断口特征、石墨结晶核心及变异石墨形态分析,研究了孕育处理对大断面QT600－3球铁件力学性能和石墨形态的影响。结果表明,电炉熔炼工艺生产大断面球铁件时,采用随流瞬时孕育,孕育衰退作用减慢,抗拉强度和伸长率提高;YFY－1A作瞬时孕育剂,石墨形态良好,石墨球大小稳定;SPI作瞬时孕育剂,导致石墨形态发生变异。     

Precipitation and evolution of nodular graphite during solidification process of ductile iron

The quantity and morphology of spheroidal graphite have an important effect on the properties of ductile iron,and the characteristics of spheroidal graphite are determined by the solidification process.The aim of this work is to explore the precipitation and evolution of graphite nodules in hypoeutectic,eutectic,and hypereutectic ductile irons by thermal analysis,liquid quenching and metallographic technique.Results show that hypoeutectic ductile iron has the longest solidification time and the lowest eutectic temperature;eutectic ductile iron has the shortest solidification time;hypereutectic ductile iron has the highest eutectic temperature.After solidification is completed,hypoeutectic ductile iron has the lowest nodule count,nodularity and graphite fraction;eutectic ductile iron has the highest nodule count,nodularity and the smallest nodule diameter;hypereutectic has the highest nodule diameter and graphite fraction.The nucleation and growth of graphite nodules in hypereutectic ductile iron starts before bulk eutectic crystallization stage,however,the precipitation and evolution of graphite nodules of hypoeutectic and eutectic ductile irons mainly occur in the eutectic crystallization stage.The graphite precipitated in eutectic crystallization of hypoeutectic,eutectic,and hypereutectic ductile irons,are 61%,68% and 43% of total graphite volume fraction,respectively.Simultaneously,there are plenty of austenite dendrites in hypoeutectic and hypereutectic ductile irons,which are prone to shrinkage defects.Therefore,the eutectic ductile iron has the smallest shrinkage tendency.     

灰口铸铁中石墨分布的优化

控制石墨的形态和分布是优化灰口铸铁性能的关键;石墨的形态取决于凝固过程中经过共晶温度区间时的冷却条件;通过选择合适的化学成分和金属液处理方法可以控制共晶凝固过程,获得分散的、均匀分布的A型石墨,从而得到性能优良的灰铁铸件。     

S、Mg和RE含量对球铁球化率及性能的影响

生产条件不同，球铁铸件性能的差异很大。分析球铁生产中S与Mg、RE的关系，以及这3个元素含量对球铁质量的影响。针对厚壁铸件中经常出现的碎块状石墨提出了相应的改进措施：适当增加碳，降低硅，提高残余Mg量，降低残余RE量，采用冷铁提高冷却速度。     

激冷球铁的正火与石墨表面结构对共析物的影响

通过对金属型离心浇注气缸套正火出现异常结果分析,发现在共析石墨化过程中,仅以石墨－奥氏体接触面的多少不足以准确反映其真实情况;反复研究、比较了几种铸铁的共析石墨化条件和结果,强调了石墨表面结构的重要性,确认了激冷球铁正火的特点,并经过试验探索,提出了改善激冷铸铁正火效果的思路。     

Improving chill control in iron powder treated slightly hypereutectic grey cast irons

Recent studies revealed that in eutectic to slightly hypereutectic grey irons (CE = 4.3%-4.5%) the presence of austenite dendrites provides an opportunity to improve the cast iron properties, as a high number of eutectic cells are "reinforced" by austenite dendrites. An iron powder addition proved to be important by promoting dendritic austenite in hypereutectic irons, but was accompanied by adverse effect on the characteristics of potential nuclei for graphite. The purpose of the present paper is to investigate the solidification pattern of these irons. Chill wedges with different cooling moduli (CM = 0.11 - 0.43 cm) were poured in resin bonded sand and metal moulds.Relative clear / mottled / total chill measurement criteria were applied. Iron powder additions led to a higher chill tendency, while single inoculation showed the strongest graphitizing effect. The various double treatments show an intermediate position, but the inoculant added after iron powder appears to be the most effective in reducing base iron chill tendency, for all cooling moduli and chill evaluation parameters. This performance reflects the improved properties of (Mn,X)S polygonal compounds as nucleation sites for graphite, especially in resin bonded sand mould castings. Both austenite and graphite nucleation benefit from a double addition of iron powder + inoculant,with positive effect on the final structure and chill tendency.     

Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings

Abstract

There is a strong demand for heavy section castings made of spheroidal graphite with a fully ferritic matrix, e.g. for manufacturing hubs for windmills. Such castings with slow solidification process are prone to graphite degeneration that leads to a dramatic decrease of the mechanical properties of the cast parts. Chunky graphite is certainly the most difficult case of graphite degeneracy, though it has long been known that the limited and controlled addition of antimony may help eliminate it. The drawback of this remedy is that too large Sb additions lead to other forms of degenerate graphite, and also that antimony is a pearlite promoter. As part of an investigation aimed at mastering low level additions to cast iron melts before casting, solidification of large blocks with or without Sb added was followed by thermal analysis. Comparison of the cooling curves and of the microstructures of these different castings gives suggestions to understand the controlling nucleation and growth mechanisms for chunky graphite cells.     

耐低温冲击球墨铸铁的工艺控制

为生产耐低温冲击球墨铸铁件,制定了不同球化及孕育处理工艺的组合方案,对浇注的试样进行了拉伸和低温冲击性能试验,并观察了显微组织,分析了不同预处理工艺和多次孕育工艺对球墨铸铁石墨形态和力学性能的影响。结果表明：采用稀土球化剂、包内预处理及多次孕育的组合工艺,能显著改善石墨球径的均匀性及石墨球分布的均匀性,保证球墨铸铁低温冲击性能的稳定。     

Occurrence and effect of casting skin in compacted graphite iron

Most iron castings retain their as cast surfaces because of their geometric complexity and to minimise the machining costs. However, the mechanical properties that are documented in standards (i.e. ASTM) are tested on fully machined test bars. Therefore, the effect of the as cast surface and subsurface features (commonly referred as ‘casting skin’) on mechanical properties should be evaluated. Preliminary works have shown the negative effect of the casting skin on mechanical properties of grey and ductile irons. This paper reviews the recent works on the casting skin effect on tensile and fatigue properties in compacted graphite and ductile irons. It was found that the tensile and fatigue strength were reduced by 9 and 40% respectively because of the presence of the casting skin. In addition, the correlations between processing parameters (e.g. nodularity and section thickness) and the casting skin features were presented. The experimental results suggest that the Mg depletion because of metal–mould and metal–air interactions was the main reason for the formation of the casting skin.     

水冷铜型凝固下球墨铸铁凸轮轴研制

介绍了球铁凸轮轴水冷铜金属型铸造工艺,为了满足球铁凸轮轴的要求(QT700-2,铁素体≤5%),适当加入合金元素Cu、Sn,其中Cu控制在1.0%～1.4%,Sn在0.03%～0.05%.球化处理时,减少Mg、RE的加入量,同时对铸件进行正火处理.经批量生产验证,该工艺稳定,效率高,可以满足球铁凸轮轴的生产要求.     

高强高韧合成球墨铸铁的组织及力学性能

以废钢为主要原材料(20%生铁+20%回炉料+60%废钢),使用中频感应电炉熔炼,采用中间加入和镜面加入联合增碳方式,制备了合成球墨铸铁QT450-23铸件。合成铸铁球化级别1级,球化率95%,石墨圆整,球径10～20μm,基体为100%铁素体。合成铸铁抗拉强度为450MPa,伸长率为23.3%。在高温组织中,奥氏体枝晶发达,显著提高材料的冲击韧性,V型缺口冲击试样常温冲击韧性为18.4 J/cm2,是相同成分和工艺条件下,传统生铁为主配方球铁的2倍。     

微量锑对灰铸铁显微组织和性能的影响及应用

通过试验证明,往灰铸铁中加入微量的锑,可以明显地细化石墨,并增加基体中珠光体的数量,从而使铸铁的抗拉强度和硬度得到明显的提高。介绍了在实际生产中应用锑来增加厚大铸件的硬度以及提高复杂铸件铸造质量的情况。     

A Study of the Microstructure,Mechanical Properties and Weldability of Low-carbon Ductile Iron

Abstract

Low-carbon cast iron (1.7–1.8% C, 1.7–2.2% Si) test bars and castings were prepared in a production foundry for evaluation of their microstructures and mechanical properties. The experimental variables were (a) inoculant types and (b) heat treatment cycles. Results indicate that low-carbon ductile cast irons with graphite present in spheroidal/compacted form may be satisfactorily produced on an industrial scale, after appropriate inoculation. However, a short normalising cum tempering heat treatment is necessary for the development of optimum mechanical properties. Low carbon irons can be satisfactorily welded to similar grades of cast irons as well as to steel.     

Effect of microstructure and mechanical properties on cutting force of different cast irons with similar tensile strength

Flake graphite iron,compacted graphite iron and spheroidal graphite iron with various tensile strengths were cast. They were selected and grouped according to roughly the same tensile strength,and then the main cutting force in each group was measured and compared. The microstructures of different cast irons were characterized. The relationship between the cutting force and microstructure was established. Results show that the graphite morphology in cast irons determines the strength. In order to obtain the same strength of the cast iron with sharply edged graphite,more or finer pearlite in the matrix is needed. Graphitic cast irons with high pearlite content and smaller pearlite interlamellar spacing have higher hardness. For the cast irons with different graphite morphologies,but almost the same tensile strength,the main cutting force is obviously different,along with the hardness. Harder cast irons have a greater cutting force,but the difference in cutting force is not proportional to hardness.     

Effect of nodularity on mechanical properties and fracture of ferritic spheroidal graphite iron

Ferritic spheroidal graphite irons with nodularity from 72% to 96% were prepared. The relationship between the nodularity and the mechanical properties of the ferritic spheroidal graphite iron was investigated. The effect of nodularity on the mechanical properties and tensile fracture of the cast iron were studied. Results showed that the tensile strength Rm, yield strength R_(p0.2), elongation to failure A_5, and impact energy KV_2 of the cast iron had a good linear relationship with its nodularity. Nodularity and annealing treatment would obviously affect the fracture characteristics of ferritic spheroidal graphite iron. The annealed ferritic spheroidal graphite iron with 93% nodularity showed a completely ductile rupture. With the decrease of nodularity from 93% to 72%, the cleavage fracture area ratio increased gradually from 0% to 8.3%. Compared with as-cast ferritic spheroidal graphite iron, annealing treatment reduced the cleavage fracture area of the ferritic spheroidal graphite iron.     

低钙钡的硅钡孕育剂

介绍一种低钙钡的硅钡孕育剂，其Si、Ba、Ca、Al组元配比合理，既具有强的石墨化作用，又具有良好的脱氧性能，因此能延缓孕育衰退时间，防止铸件局部白口，使铸件硬度均匀。用于铸态球铁，可以获得足量的铁素体；用于灰铸铁，可以稳定获得A型石墨，石墨长度达4～6级，力学性能提高。